“They just wanted to make sure the paperwork was correct and things were happening as they should be happening, and that’s what they found, that nothing was untoward.”

MPI director of readiness and response Geoff Gwyn told RNZ’s Checkpoint that investigations had revealed that Mycoplasma bovis could have been present as early as December 2015.

“MPI conducts a full investigation into all the farms that are under regulatory control… but I’m not going to go into detail into any of those investigations.

“The animal welfare responsibility sits with the farmer so the farmer is obviously in the best position to understand the health of their herd, so it’s up to them to notify their vet if there are any concerns.”

Infected milk looks to be one of the reasons Mycoplasma bovis has spread to 38 farms from Southland to Waikato.

Waste milk is sold by dairy farmers to those looking for a cheap way to feed their calves. But milk from cows with the cattle disease is infectious and the calves it is fed to are vulnerable.

Southland vet Mark Bryan led a study that found such milk was sold to farms up to 130 kilometres away from the source farm.

He wanted to see a system introduced to track the movement of waste milk.